Tiny salt crevices could act as miniature ‘Greenhouses’ for Martian microbes

We’ve been searching for life on Mars for a long time, but so far we’ve come up empty. The fundamental problem is a Catch-22 involving two of the key ingredients for life. Life as we know it needs liquid water and reasonable temperatures, both of which are available on Mars, but hardly ever at the same time. But a new paper from lead author Anna Bognar and her team at ELTE Eötvös Loránd University and the Konkoly Observatory describes a way for life to access both requirements at the same time—by hiding away in salt crystals.

To understand why, let’s first look more closely at the Catch-22. During the Martian night, the planet’s air cools and its relative humidity climbs. Salts scattered across the surface can absorb some of that vapor through a process known as deliquescence. However, it is also much colder than any metabolizing organism could survive—dropping to -80℃, well out of range of life as we know it.

On the flip side, during the Martian day, temperatures warm to a much more reasonable 0℃, or even higher at some points near the equator. That’s well within the operational range for known life forms—after all, Canadians deal with temperatures colder than that for almost half the year. However, during this warm period, the sun also burns off all the moisture, driving relative humidity down to near zero. So while it’s warm, all the water trapped overnight is suddenly gone.

However, one feature of those massive temperature swings, which can reach 150℃ in a single day, can also be used to trap water. Materials contract and expand as they are cooled and warmed, creating a serious amount of mechanical stress on rocks undergoing that shift. For example, according to the authors’ models, halite (NaCl) salt crystals can contract by up to 1.6% between 150 K and 300 K, which can potentially open microscopic fractures and crevices.

At that coldest point, in the dead of night, relative humidity also peaks, and the “hygroscopic” (i.e. water-absorbing) salt pulls moisture out of the air, forming microscopic brine.

In the morning, the sun heats that same material, the crystal expands and it swells back to its original size, choking off the microfractures that formed during the night. That means that while the outer surface of the material dries out, water could remain inside those microscopic channels, becoming liquid in the heat but unable to escape into the wider atmosphere because there is no way out of the crevice.

That means, for a precious few hours every day during the summer, at least, those microscopic cracks are filled with liquid water at temperatures where known biology can operate effectively. But the sealed crack provides other benefits, too. Even a few millimeters of material sealing over the crack could provide protection from deadly UV rays, which would otherwise sterilize any type of unprotected life on the Red Planet. But it also allows enough visible light through that, at least in theory, any microbes existing in those cracks could harvest energy through photosynthesis.

In fact, there are already examples of bacteria doing something like this on Earth. In Chile’s Atacama Desert, the driest place on Earth, microscopic cyanobacteria have completely left the soil and instead taken up residence inside the pores of halite rocks, surviving on the water the salts in those rocks pull out of the air. So, in practice, at least one side of the equation works for life as we know it.

But there are still some challenges to address. Obviously, there’s no guarantee that a crevice would seal completely every day, allowing some water to trickle out during the day and potentially creating a “feast or famine” cycle for microbes attempting to scrape out a living inside it. And the salt itself could be filled with perchlorates, which are deadly to most forms of Earth-based life. Finally, while microbes in these crevices may be protected from UV radiation, they aren’t protected from higher-energy cosmic rays, which care little for a few millimeters of protective covering.

All that is to say—while it might be theoretically possible for microbes to survive in such an environment, it is certainly unlikely. But, as Jeff Goldblum famously put it in Jurassic Park, “life, uh, finds a way.” And maybe it found a way to survive the Red Planet’s desiccation by hiding in salt crevices. If it has, we might someday find it and, in so doing, answer one of life’s great mysteries—knowing that we’re not alone.

The paper is published in the journal Icarus.

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